In a gas turbine engine each main shaft must be located by at least two bearing assemblies. One may comprise ball bearing elements 10 carried between an inner raceway 12 and an outer raceway 14, with a cage 16 separating the bearings, where the cage rotates with the ball bearings, for example as shown in FIG. 1. Such bearing assemblies are able to take axial and radial loads. The other bearing assembly of the pair may comprise roller bearing elements 20 with an outer raceway 24 and cage 26, with the roller elements 20 bearing directly onto a shaft 28 they support, for example as shown in FIG. 2. The roller bearing withstands radial loads but permits the shaft to move axially without impediment so that thermal expansions can be accommodated. The region of the shaft upon which the bearing runs may be hardened, for example by a heat treatment or other technique for altering the material properties of the shaft material in that region.
In such an arrangement, engine design normally dictates that the ball bearing arrangement (i.e. the FIG. 1 example) that resists the axial and radial loads is in a relatively cool region for example, in the compressor region of a gas turbine engine) so as to limit thermal expansion and contraction and thereby limit thermo-mechanical loads on the bearing assembly. Since the roller bearing is less constrained axially, and hence better able to cope with thermo-mechanical loads induced by thermal expansion and contraction, the roller bearing tends to be located within the relatively hotter turbine region.
It is difficult to deliver and scavenge (i.e. remove) oil to the turbine region for lubrication and cooling because of build constraints and operating temperature of the turbine. Often, whilst the supply of oil can be maintained the scavenge is badly affected by thermal degradation of the oil.
Hybrid ball bearings are in use in machine-tool ball bearings where the low weight and high stiffness make the balls ideal for high speeds and low heat generation. “Hybrid” is a term used within the bearing industry to identify a bearing with metal or steel inner and outer raceways that uses ceramic rolling elements.
Hybrid roller bearings (i.e. bearings with a metal outer race and ceramic roller bearing) are expensive and difficult to manufacture. Both the roller bearings and the required cage require complex machining. In operation, although roller bearings under high speeds need little in the way of lubrication for the rolling of the rollers, it is necessary for the rollers to be guided by ribs 18, where the ribs for a roller bearing require a significant amount of lubrication.
Also the cooling of the bearing assembly within the turbine structure aggravates thermal stresses within the bearing assembly. The outer raceway is in contact with metal washed by hot gasses, which makes the outer raceway of the bearing structure hot. Conversely the inner raceway of the bearing structure is maintained at a relatively low temperature by the bearing lubricating oil and because it is in contact with a relatively cool structure. Thus any of the bearing assemblies described above will suffer considerable thermal mechanical stress because of the thermal gradient across them. Additionally, the wide range of temperatures experienced by the bearing assembly means that the outer bearing race will expand and contract relative to the inner bearing race (where present), the caged bearing elements and the shaft they support. Thus under some temperature conditions and thermal gradients, and with respect to the example shown in FIG. 1 above, the ball elements may not be in contact with, or may only lightly touch, the inner race. Likewise, with respect to the example shown in FIG. 2 above, the roller elements may not be in contact with, or may only lightly touch, the shaft under some temperature conditions and thermal gradients. Thus the bearing elements in both examples may skid (i.e. not achieve their epicyclic speed) and thus slide on the surface of the race and/or shaft, increasing frictional losses and increasing wear on the element, race and/or shaft, and thus reducing the operation life of the arrangement.
Hence a bearing assembly which is relatively simple and cheap to manufacture, which is operable in a high temperature environment, has a low lubrication requirement and is tolerant of high temperature gradients, is highly desirable.